The exchange of digital data between ground stations and a communication satellite may be based on the principle of time-division-multiplex access (TDMA), utilizing several channels of different carrier frequencies each serving for the transmission of messages from a plurality of ground stations in the form of interleaved bursts separated by intervening guard intervals. Aboard the satellite, the data bursts of an incoming message frame are processed for retransmission to their respective destinations in an outgoing message frame of similar structure; the on-board processing includes demodulation of the incoming carrier, regeneration and re-amplification of the base-band signals, and modulation of these signals upon an outgoing carrier. These operations necessarily proceed under the control of a time base aboard the satellite; the bit cadence of the incoming signals, however, is determined by the clocks of their respective ground stations.
Even if the time base of the satellite is controlled by a ground-station clock with the aid of synchronizing signals, its clock frequency will generally deviate from that of the ground station to a greater or lesser extent, e.g. on account of operational delays, Doppler shifts due to relative motion between the satellite and a ground station, and phase differences between the clocks of ground stations accessing the same channel. As a result, distortion or loss of information may occur when the bits received aboard the satellite are read in a rhythm deviating from their rate of arrival.
A possible solution to this problem lies in the use of a buffer memory designed to store a certain number of incoming bits (e.g. three) in respective stages from which they can be read out at a rate different from their rate of loading. This solution, however, necessarily delays the transmission of acknowledgment signals from the satellite to the originating ground stations which serve for the correction of excessive phase shifts between the signal stream emitted by that station and the window allocated to the station in the transmitted message frame. Such a delay, which may amount to the full storage time of the buffer memory, would necessitate a lengthening of the guard intervals between successive bursts in order to prevent interference between messages from different stations; this, in turn, would reduce the capacity of the channel.